Due to the jump of fossil energy price in recent years, it is required the system which can generate electric power directly from natural energy. To respond such requirement, it have been proposed or put to practical use a solar cell using Si in a single crystal, a polycrystalline, or an amorphous state, a solar cell made of a compound of GaAs or CIGS, or a dye sensitized photoelectric conversion element (Gratzel cell).
However, the cost of electric power generation with these solar cells is still higher than the price of electricity generated and transmitted using the fossil fuel, and it has been an obstacle for the spread of solar cells.
Moreover, it is needed to use a heavy glass as a substrate, and reinforcement work is required at the time of installation. This is also a cause which increases the cost of electric power generation.
Under such circumstance, it was proposed a bulk hetero junction type photoelectric conversion element as a solar cell which can achieve lower electric generating cost than the electric generating cost using a fossil fuel. In this photoelectric conversion element, there is a bulk hetero junction layer mixed with an election donor layer (p-type semiconductor layer) and an electron acceptor layer (n-type semiconductor layer) sandwiched between an anode and cathode (for example, refer to Non-patent document 1 and Patent document 1).
In these bulk hetero junction type solar cells, the composing members are formed with a coating process except an anode and a cathode. Therefore, it is expected that high-speed and low cost production is possible, and the problems of the above-mentioned electric power generation cost may be resolved. Furthermore, unlike the above-mentioned Si system solar cell, compound semiconductor system solar cell, and dye-sensitized solar cell, there is no high temperature process above 160° C. Therefore, it is expected that formation of a cell on a low cost and lightweight plastic substrate is also possible.
In addition, the electric power generating cost must be calculated by including generation efficiency and durability of an element besides an initial manufacturing cost. In the above-mentioned non-patent document 1, in order to absorb a natural sunlight spectrum efficiently, it has come to attain the conversion efficiency exceeding 5% by using an organic polymer which can absorb to about 900 nm.
However, in order to obtain a further improved efficiency, utilization of natural sunlight with broader spectra is requested. In order to use natural sunlight with broader spectra, it is requested to expand the wavelength band which can be used or raising the absorbance of specific wave length.
As a technology to achieve both improvement of the absorbing efficiency of light and expanding the wavelength band, the tandem technology has been paid attention. This is a technology which laminates two or more solar cells. If the solar cells to be laminated are the same, increase of the light absorption efficiency will be anticipated, and if the solar cells which absorb different wave length are laminated, expansion of the wavelength band which can be used will be anticipated.
As an example of such technology, a solar cell system is disclosed in which one solar cell is connected to another solar cell having the completely same composition composed of phthalocyanine and N,N-dimethylperylene tetracarboxylic diimide with very thin gold (for example, refer to Non-patent document 2). As a result, although improvement in open circuit voltage is obtained, the photoelectric conversion efficiency itself is falling from the fall-off of the short-circuit current accompanying a layer upon layer. The solar cell located at the inner part from the incidence side of a light will surely receive a decreased amount of light and the amount of generated electric current will be decreased. Furthermore, with a tandem element, since an electric current value is restricted to lowest electric current value in the solar cell system by which the stack was carried out, it is presumed that improvement in efficiency was not obtained. From such a viewpoint, it is thought preferable that the solar cells to be laminated each absorb a different spectrum with each other. Moreover, since vacuum evaporation is employed to produce an element, manufacturing efficiency is low.
There is disclosed a tandem-type solar cell which has a first layer containing polyphenylene vinylene (PPV) and phenyl-C61-butyric acid methyl ester (PCBM), and a second layer containing poly(3-hexylthiophene) (P3HT) and PCBM (for example, refer to Non-patent document 3). Absorption of PPV and P3HT each is around 600 nm, and since they absorb almost the same natural sunlight spectrum, efficiency has not been improved by making a tandem-type composition compared with single composition.
Further, there is disclosed a tandem-type solar cell which has a first layer containing poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopentadithiophen)-4,7-(2,1,3-benzthiadiazole)] (PCPDTBT) and PC60BM, and a second layer containing P3HT and PC70BM (for example, refer to Non-patent document 4 and Patent document 2). The composed element having a higher efficiency than an efficiency of a single cell can be obtained at last here. It has been succeeded in obtaining a photoelectric conversion element having a high efficiency of 6.5% by designing the portions of an absorption spectrum so that the 1st layer mainly absorbs lights of 600 to 900 nm and the 2nd layer mainly absorbs lights of 400 to 600 nm.
However, neither of the disclosed tandem-type organic photoelectric conversion elements (for example, refer to Non-patent documents 3 to 5) reaches completely the efficiency of the sum of the efficiency of single composition, and the ideal tandem element is not yet obtained.
As a possible reason of this, it is presumed as follows. When a tandem composition is achieved, although an open circuit voltage (Voc) is obtained as a sum of each single element, decrease of a short-circuit current (Jsc) and a fill factor (FF) is observed, namely, the resistance as the whole element is increased.
Moreover, when putting an organic thin film type solar cell in practical use, not only efficiency but cost and durability become three important components. Regarding the durability, it was disclosed that the relative efficiency fall-off with respect to the initial efficiency was as large as about 40% after 100 hours use under the optical exposure of AM1.5G. This indicates that there is still problem of durability.
The present inventors assumed the reason of such an efficiency fall-off as follows. As a result by which the production of electricity from two bulk hetero junction layers is not coincided thoroughly, as opposed to the requirement that a hole which comes from one bulk hetero junction layer and an electron which comes from another bulk hetero junction layer should be recombined mutually to result in becoming a neutral electric charge, it was presumed that the electric current (electrons) from the 1st bulk hetero junction layer that mainly absorbs a short wave region becomes overabundant, and the electrons which cannot be recombined form reverse jointing at the interface with the 2nd bulk hetero junction layer, to result in forming resistance. Moreover, it was assumed that this will be concerned not only with a mere efficiency fall-off but with deterioration of an organic photoelectric conversion element.